The synthesis of low oxidation state technetium (Tc) and rhenium (Re) radiopharmaceuticals has become feasible recently because of the ease with which fac-[M(CO)3X3]n (M=Tc, Re; X=Cl or Br, n=−2; X=H2O, n=+1) can be prepared at the tracer level,1 using 99mTc (γ, t1/2=6.02 h), 186Re (β−, t1/2=91 h) and 188Re (β−, t1/2=17 h), and on a macroscopic scale2 using 99Tc (β−, t1/2=2.12×105 yr) and 185,187Re. Consequently, several publications describing the preparation of technetium(l) and rhenium(l) radiopharmaceuticals have appeared in the literature.3 These complexes are typically composed of bidentate4 or tridentate chelates5 or organometallic ligands.6 
There has been particular interest lately in preparing substituted η5-cyclopentadienyltricarbonyl rhenium and technetium complexes because of their unique physical and chemical properties compared with Tc(V) and Re(V) coordination complexes, which, at present, are more commonly used to develop new radiopharmaceuticals. The main obstacle to using cyclopentadiene (Cp) as the core of bifunctional radiopharmaceutical ligands has been the lack of a mild and direct method for the synthesis of the metal complexes, which is readily adaptable for use at the tracer level. Top et al.7 reported an elegant method for preparing rhenium(l) complexes of substituted cyclopentadienes through an exchange reaction with cyclopentadienyltricarbonyl manganese derivatives while Katzenellenbogen's8 group has reported a one pot procedure for preparing cyclopentadienyltricarbonylrhenium through the in situ generation of Cp-trialkylstannane derivatives. The involvedness of the procedures and, in the latter case, the poor yields of synthesis in concert with the required use of potentially toxic reagents, at present, limit the widespread use of Cp as a bifunctional radionuclide ligand.
The carborane, 7,8-dicarba-nido-undecaborate, when deprotonated, has an open pentagonal face that can form metallocene-type sandwich complexes with analogy to the formally isolobal Cp− ion.9 As a result, η5-carborane complexes with a variety of metals have been reported, two of the earliest being [Re(CO)3(η5-7,8-C2B9H11)]− (1, FIG. 1) and [Mn(CO)3(η5-7,8-C2B9H11)]−.10 The original method for the preparation of [Re(CO)3(η5-7,8-C2B9H11)]−involved reacting [nido-7,8-(C2B9H11)]2−, the dicarbollide dianion, with Re(CO)5Br. Ellis et al. reported an improved synthetic procedure starting with [ReBr(CO)3(THF)2] prepared in situ from ReBr(CO)5 under anhydrous conditions.11 The corresponding reaction with technetium was never reported, which may have been a consequence of radioactivity issues and/or the fact that the synthesis of Tc(CO)5Br is not uncomplicated. Convenient methods for preparing M(CO)5Br (M=99mTc and 186,188Re) in aqueous solutions at the tracer level suitable for routine radiolabeling experiments do not, at present, exist.
There is a need for new methodologies for the synthesis of radiopharmaceuticals, in particular radiopharmaceuticals of η5-carborane complexes of technetium and rhenium, that are suitable for routine radiolabeling experiments (i.e. can be performed in aqueous solutions at tracer levels).